This invention relates to stabilizing halogenated polymers and to an improved method for stabilization thereof. More particularly, the invention relates to thermal stabilization of halogenated polymer resins that are normally processed at elevated temperatures into formed articles. In particular, the invention relates to thermal stabilization of such resins with certain synthetic crystalline aluminosilicates or zeolite molecular sieves prepared specifically for this purpose.
Halogen containing polymers, such as polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) and other chlorine, fluorine, or bromine containing polymers which are formed above about 150xc2x0 C. tend to degrade and discolor unless stabilized, even when held at these temperatures for the relatively short period of time required for processing. When this occurs, the polymer is either unusable or a high rejection rate for produced articles results, and the ability to reuse scrap material from the formation process is seriously impaired.
Subsequent to processing such resins, articles made from such halogenated polymers, unless stabilized and properly pigmented, also tend to deteriorate, become brittle and crack or shatter upon prolonged exposure to sunlight or other ultraviolet rays, thereby becoming useless for their intended purpose.
These resins in their unstabilized form also tend to plate out on processing equipment, for example on dies and mill or calendar rolls, as the material is being formed into a finished article. This causes imperfections in articles made from such resins and tends to require frequent cleaning of processing equipment used for handling such materials, resulting in inefficiencies in production as well. For example, processing of unstabilized PVC at elevated temperatures rapidly results in degradation whose symptoms are discoloration, elimination of hydrogen chloride, and irreversible adhesion to the processing equipment surfaces.
While all mechanisms for these instabilities are not precisely known, the thermal degradation and perhaps the other indicated forms of instability of halogenated polymers, particularly polyvinyl chloride (PVC) or chlorinated polyvinyl chloride (CPVC), is manifested in the evolution of HCl. It is widely known that HCl catalyzes further degradation. Prevention of this degradation requires keeping HCl at very low concentrations and/or neutralizing it during processing.
Traditional methods for stabilizing halogenated polymers such as PVC have focused on the use of various inorganic, organometallic and organic stabilizers. Inorganic stabilizers that have been used include for example dibasic, tribasic, and tetrabasic lead sulfate; dibasic lead phosphite; and white lead. Organometallic stabilizers commonly used include organic adducts of such heavy metals as barium, cadmium, lead, tin, magnesium, antimony, and/or zinc, frequently in admixture with other co-stabilizers and other conventional additives. Organic stabilizers that have been used include for example, calcium soaps, polyhydric esters of various fatty acids, phosphites, thioesters, beta-diketones and the like, alone or in combination with such organometallic compounds as stabilizers for such resins.
Aluminosilicates or zeolite molecular sieves have also been suggested for use as stabilizers for PVC. For example, U.S. Pat. No. 3,245,946 discloses the use of activated Zeolite A as a stabilizer for PVC resin. And U.S. Pat. No. 4,000,100 discloses the use of an unactivated Zeolite 3A, 4A, or 5A molecular sieve in combination with a conventional organometallic or organic stabilizer mixture. It is also known to utilize a complex system of primary and secondary stabilizers, including as one component of the stabilization system, a powdered, crystalline, synthetic hydrous aluminosilicate having a water content in the range of 13 to 25% as water of crystallization, as disclosed in U.S. Pat. No. 4,590,233.
Among the prior art utilizing zeolites or aluminosilicates as part of a stabilization system for PVC resin, focus has been on such factors as the level of water of crystallization, and/or on pore size of the aluminosilicate. Preferred particle size ranges of zeolites are also described. Presumably certain size ranges give optimal dispersion and enhanced physical properties such as tensile strength and modulus as is known with other solid polymer additives such as calcium carbonate.
However, we are not aware of any prior art disclosure that establishes or suggests a direct correlation between reduction in particle size and enhancement of thermal stability. Nor are aware of any significance attached in prior art disclosures to the distinction made with respect to this invention between particle size and crystallite size. Conventional zeolites consist of small cubic or prismatic crystallites and/or other geometric forms such as rhombic, dodecahedral, spherulite, octahedral, etc., and combinations and intergrowths thereof, that agglomerate into particles. The degree of agglomeration and/or inter-growth determines the particle size distribution, which is typically determined by a light scattering or other spectroscopic technique, whereas crystallite size is virtually independent of particle size and is typically determined by scanning electron microscopy images. As an example of this distinction, zeolite 4A and faujasite-type zeolites currently offered on the market typically have a crystallite size of about 1.0 to 5.0 microns with a mean particle size of about 3.0 to 10.0 or more microns. Moreover, while the prior art does recognize methods for reducing particle size, it does not discuss preparative procedures for the small crystallite zeolites as claimed in the present application.
While the use of factors such as complex stabilization systems and activation of zeolites has resulted in substantial improvement in the ability of aluminosilicates to stabilize PVC resins, still further improvements are required, particularly in the level of thermal stabilization for PVC, CPVC, and other halogenated polymers.
Accordingly, it is the primary object of this invention to provide an improvement in the thermal stability of halogenated polymer molding resins such as PVC, CPVC, and other halogenated polymers utilized for forming articles at elevated temperature.
It has now been found that these and other improvements in the stability of halogenated polymer resins are obtained in accordance with the present invention by utilizing as a stabilizing component of such halogenated polymers an aluminosilicate in which the individual crystals have a fine crystallite size in the range of about 0.01 xcexcm to about 1.0 xcexcm. Surprisingly it has now been found that while particle size is important in the maintenance of physical properties of halogenated polymer systems, utilization of an aluminosilicate having a fine crystallite size is critical to obtaining further improvements in thermal stabilization of these polymers. Such fine crystallite particle size stabilizers will normally be employed in combination with conventional inorganic, organometallic, or organic co-stabilizers.
Additionally, it has been found that, for example, in CPVC and rigid PVC formulations where relatively higher concentrations of aluminosilicate are needed than for flexible PVC applications, a particle size in a range of about 0.5 to about 3 xcexcm is preferred. Furthermore, maintaining a relatively low degree of hydration of the aluminosilicate is important to prevent the creation of water bubbles. Steam calcination dehydration of the aluminosilicate to a water content of less than about 8% water by weight of the aluminosilicate prevents rehydration of the aluminosilicate above a water content of 10%. Such steam calcined dehydrated zeolites are particularly well suited for preventing or minimizing water bubbles when used in formulations with relatively high concentrations of zeolite.
In general, the present invention comprises a stabilized halogenated polymer composition comprising a halogenated polymer resin and a stabilizing amount of a synthetic crystalline aluminosilicate having a fine mean crystallite size in the range of about 0.01 xcexcm to about 1.0 xcexcm and preferably a mean crystallite size in the range of about 0.2 to about 1 xcexcm and most preferably in the range of about 0.2 to about 0.85 xcexcm. While crystallite size is critical to the present invention, particle size may generally vary within a wide range, but it is preferable to employ particles having a mean particle size in the range of about 0.1 to about 5.0 xcexcm, and more preferably, especially for CPVC and rigid PVC formulations, about 0.1 to about 3 xcexcm. Also included in the present invention is a method for stabilizing such compounds by admixing the halogenated polymer with the foregoing aluminosilicate, alone or in combination with conventional inorganic, organometallic, and/or organic co-stabilizers.
Polymer resins suitable for use in the present invention are halogenated polymer resins including but not limited to, polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polychloroprene (Neoprene), an acrylonitrile-vinyl chloride copolymer (Dynel), a polyvinyl chloride-vinyl acetate copolymer, polyvinylidene chloride, a polytetrafluoroethylene-vinyl chloride copolymer, or a polyfluoro-chloro ethylene polymer, most preferably a halogenated vinyl polymer resin. Polyvinyl chloride and chlorinated polyvinyl chloride are particularly preferred resins for use in the present invention. The present invention may be used in both rigid and flexible PVC formulations. As used herein, a xe2x80x9crigidxe2x80x9d formulation is defined as having a plasticizer or combination of plasticizers and/or elastomers in a concentration of zero to less than about 5 parts per hundred of resin (phr). A xe2x80x9cflexiblexe2x80x9d formulation typically has from greater than about 5 to as much as about 150 phr plasticizer(s). CPVC formulations are typically rigid only.
The PVC used for this invention may be one of several types; its properties can vary depending on the polymerization method and polymerization temperature. The higher the polymerization temperature, the lower the molecular weight of PVC that is produced. Typical PVC resins have a number average molecular weight in the range of about 20,000 to about 100,000 and a Fikentscher K value ranging from about 41 to about 98. Certain molecular weight PVC resins show optimum physical properties and process performance for particular applications. Generally, suspension PVC resins are more thermally stable than PVC resins produced by mass or emulsion polymerization. Depending on the polymerization method and manufacturing conditions, PVC can vary in resin particle size and porosity and may perform differently in the same formulation.
The aluminosilicates used in the present invention are synthetic crystalline aluminosilicates, commonly called xe2x80x98zeolites.xe2x80x99 Zeolites are materials with discreet channels and cages that allow the diffusion of small molecules into and out of their crystalline structures. The utility of these materials lies in their microstructures that allow access to large internal surface areas and that increase adsorptive and ion exchange capacity.
The aluminosilicates or zeolites useful in the present invention may be generally designated by the chemical formula M2/nO.Al2O3.ySiO2.wH2O in which M is a charge balancing cation, n is the valence of M and is 1 or 2, y is the number of moles of SiO2 and is about 1.8 to about 15, and w is the number of moles of water of hydration per molecule of the aluminosilicate.
Suitable charge balancing cations represented by M in the formula include such cations as sodium, potassium, zinc, magnesium, calcium, tetra-alkyl and/or -aryl ammonium, lithium, NH4, Ag, Cd, Ba, Cu, Co, Sr, Ni, Fe, and mixtures thereof. The preferred cations are alkali metal and/or alkaline earth metal cations, with the proviso that, when M is a mixture of alkali or alkaline earth metals comprising sodium and potassium and/or calcium, the preferred potassium and/or calcium content is less than about 35% by weight of the total alkali or alkaline earth metal content.
While the number of moles of SiO2 per molecule of aluminosilicate, represented in the formula by y, may be in the range of about 1.8 or greater, it is suitably about 1.85 to about 15, more suitably about 1.85 to about 10, preferably in the range of about 2 to about 5, and with respect to certain embodiments described below, in the range of about 1.8 to about 3.5.
The number of moles of water in the zeolite as water of hydration, represented in the formula by w, is suitably greater than about 0. 1, more suitably in the range of about 0.1 to about 10.
The zeolite framework is made up of SiO4 tetrahedra linked by shared oxygen atoms. Substitution of aluminum for silicon creates a charge imbalance that requires a non-framework cation to balance the charge. These cations, which are contained inside the channels and cages of these materials, may be replaced by other cations giving rise to ion exchange properties. The water in these materials may typically be reversibly removed leaving the host structure intact, although some framework distortion may occur. In addition, these materials are typically alkaline. Suspensions of low SiO2/Al2O3 ratio materials in water often give rise to a pH greater than 9. This combination of high alkalinity and the pore structure of these compounds is believed to be largely responsible for the ability of these zeolites to stabilize halogenated polymers by neutralizing acids released during processing and creating inert salts and/or scavenging excess cationic metals.
Zeolites are frequently categorized by their crystalline unit cell structure (See W. M. Meier, D. H. Olson, and Ch. Baerlocher, Atlas of Zeolite Structure Types, Elsevier Press (1996) 4th ed.) Those suitable for use as stabilizers in the present invention include compounds characterized as zeolite A, zeolite P, zeolite X, and zeolite Y. While other zeolites may also be useful in the present invention, the preferred aluminosilicate is zeolite A.
Preferably, there is employed a zeolite which is substantially anhydrous; that is, an aluminosilicate in which much of the water of hydration has been removed by dehydration prior to incorporation into the halogenated polymer formulation. Such products are frequently referred to by those skilled in the art as xe2x80x9cactivated zeolites.xe2x80x9d Suitable activated zeolites particularly useful in the present invention are those which have been dehydrated to a level at which the water content thereof is in the range of about 0.1% to about 20%, advantageously in the range of about 0.5% to about 18% and most conveniently between about 1% and about 15%, by weight of the aluminosilicate. In a preferred embodiment as described below, the zeolite is steam calcined to a water content of less than about 8% by weight of the zeolite.
It is also desirable that the aluminosilicate have a mean particle size in the range of about 0.1 to about 10 microns, suitably wherein at least about 90% of the particles are less than about 50 xcexcm, advantageously less than about 25 xcexcm, and most suitably less than about 10 xcexcm. It is also desirable that the aluminosilicate have an mean micropore diameter in the range of about 2.8 to about 8 angstroms, and/or an external surface area in the range of about 3 to about 300 m2/g.
As previously indicated, the above identified aluminosilicates may suitably be formulated with the halogenated polymer and with a co-stabilizer conventionally employed for stabilization of PVC resins. Suitable stabilizers include an organometallic complex such as magnesium, antimony, cadmium, barium, tin, calcium, zinc and/or lead, an organic complex such as polyhydric esters of various fatty acids, xcex2-diketones, organic phosphites, hindered amines, organic mercaptans, epoxidized oils, epoxy compounds and/or phenols, inorganic complexes such as dibasic, tribasic and/or tetrabasic lead sulfate, dibasic lead phosphite, and/or white lead, and/or various combinations of such organo metallic, organic and inorganic complexes.
In accordance with the method aspect of the invention, there is provided a method for stabilizing a halogenated polymer resin comprising compounding the resin with a stabilizer comprising a synthetic crystalline aluminosilicate of the formula M2/nO.Al2O3.ySiO2.wH2O, in which M, n, y and w are as defined above, wherein said aluminosilicate has a mean crystallite size in the range of about 0.01 xcexcm to about 1 xcexcm. In carrying out the process aspect of this invention, the aluminosilicate may be admixed with the polymer during production by admixing the aluminosilicate with monomers of the resin before, during or after polymerization. As with the composition aspect of this invention, it is also preferred that the aluminosilicate have a mean particle size in the range of about 0.1 to about 10 xcexcm.
The amount of zeolite added is dependant upon the application, the type of resin employed, and the formulation. For example, in most flexible PVC formulations having a relatively high amount of plasticizer, an effective amount of zeolite typically may be in a range of about 1 phr or less, but more may be used if desired. In rigid PVC and in CPVC formulations, where little or no plasticizer is present, an effective amount of zeolite typically ranges from about 0.05 to about 10 phr. As noted above, as the amount of zeolite is increased, zeolite particle size becomes more critical because of potential impact on the physical properties of the plastic. For example, larger particle size zeolites may negatively affect the impact resistance and tensile and flex strength. It is desirable in rigid PVC and in CPVC applications, therefore, to use smaller particle size zeolites that readily disperse in the polymer formulation. Because it is desirable to reduce particle size without fracturing the zeolite crystallite structure, the acceptable particle size lower limit is limited by the crystallize size. Thus, when smaller particle size zeolites are desired, it is also preferred to use zeolites with relatively smaller crystal sizes.
While degree of hydration, nature of the exchangeable cations, selection of co-stabilizer(s), and pore diameter are known to be important considerations in the ability of aluminosilicates to impart thermal stability to halogenated polymers, crystallite size has generally been found to be the most critical factor in obtaining further improvements in the ability of aluminosilicates to impart further thermal stability. As noted above, however, in applications where increased concentrations of zeolites are used, particle size is also critical. Furthermore, in such applications the degree of hydration also becomes more critical. For example, in extrusion operations, concentrations greater than about 1.2 phr of zeolite were found to evolve water bubbles in the extrudate.
It is known to dehydrate zeolites by a number of processes to reduce the degree of hydration. We have found, however, that steam calcination under certain conditions is preferred, as the resulting dehydrated zeolite will not substantially rehydrate following such dehydration. The severity of a steam calcination process is well known in the art to be dependent upon time, temperature, steam content, and pressure. It is also known that sufficiently severe steam or air calcination will destroy the crystalline structure of zeolites. On the other hand, relatively mild steam calcination conditions allow the zeolite structure to be de-aluminated and still retain zeolite crystallinity, but also leave the zeolite with the ability to re-hydrate. We have found subjecting low silica-to-alumina ratio zeolites to moderate steam calcination conditions minimizes deterioration of the crystalline structure of the zeolite and provides a dehydrated zeolite that does not significantly rehydrate. Such steam calcined dehydrated zeolites are ideal as co-stabilizers for halogenated polymers. A calcination temperature of about 400 to about 700xc2x0 C. using a steam percentage of about 20% to about 100% steam, for a time and at a pressure sufficient to dehydrate the zeolite to a water content of about 8% or less by weight of the zeolite while maintaining at least 50% of the crystallinity of the zeolite, has been found to prevent rehydration of the dehydrated zeolite to a water content of more than about 10% by weight of the zeolite. For example, 50% steam at 650xc2x0 C. for 1 hour at atmospheric pressure has been shown to be effective. It is also contemplated that 100% steam at 400xc2x0 C. for approximately 1-5 hours, or conversely 20%-80% steam at 700xc2x0 C. for 15 minutes to 1 hour may be also be effective.
Surprisingly, simply grinding larger particle size material into a finer particle size alone does not contribute significantly to improvements in thermal stability. Rather the size of the crystallite itself, as opposed to the aggregated particles of crystallites, must be reduced to within the above stated range in order to obtain these improvements in stability. Although zeolite crystal size alone is critical in flexible PVC applications where stability requirements are typically lower, for applications where stability is more important, such as in rigid PVC and CPVC applications where higher concentrations of zeolite are desired, smaller mean zeolite particle size also contributes to improved performance, as described above. Thus, for relatively higher concentrations of zeolite, both particle size and crystallite size are important, and grinding larger particle size material into finer particle sizes, in conjunction with the small crystallinity, provides the desired improved performance. It is desirable to carefully carry out the particle size reduction step, however, to prevent or minimize fracturing of the fine crystallites obtained in accordance with this invention.